The bar code structures described in this specification typically contain encoded information in a patterned format and are variously referred to as "non-volatile electro-optical read-only memories comprising symbols printed or otherwise inscribed on a substrate", "bar code symbologies", "machine readable data file structures including code words", "machine readable symbols", or simply "bar code labels".
There are two types of bar code structures: linear and two-dimensional. Linear bar code structures include constant width strips called modules. Each data element (e.g., a number or letter) is made up of a plurality of bars and spaces each of which is made up of one or more constant width modules of the same color. A scanner then "reads" the sequencing and width of the bars and spaces to decode the data.
Two-dimensional bar code structures, on the other hand, are made up of a two-dimensional array of individual square or rectangular light or dark elements. Thus, a "T" shaped symbol may denote one number while an "L" shaped symbol may denote a different number or letter.
The advantage of two-dimensional bar code structures over linear one dimensional bar code structures is their ability to store significantly more information. The disadvantage of two-dimensional bar code structures is that they require a "finder pattern" to allow the scanner to recognize each individual symbol area and to distinguish a symbol from other markings, an orientation pattern to allow the scanner to determine the physical orientation of each symbol, and a horizontal position reference pattern which allows the scanner to determine the location of each element so it can sample its state, light or dark.
This requirement of a finder pattern, an orientation pattern, and a horizontal position reference pattern results in significant "overhead" which has a detrimental impact on the goal of two-dimensional bar code structures, namely to provide a high capacity and high density symbology. In other words, the finder pattern, the orientation pattern, and the horizontal reference pattern take up a significant amount of space on the bar code label thus detracting from the anticipated higher capacity and higher density symbology.